The short answer is that is unfavorable for this to happen. Glucose does polymerize because it is favorable.

The energetics of the reaction are favorable because glucose has a reactive hydroxyl group that is made reactive by the fact that glucose in the 5 carbon ring with an oxygen bound up is a hemiacetal. This make the hydroxyl group reactive and thus it reacts with another carbohydrate with the loss of the hydroxyl group from the first carbohydrate and a hydrogen from the second carbohydrate to yield water (classic condensation reaction). The fructose in question is an acetal and thus only has no reactive hydroxyl. Fructose can only be reacted with as in the classic case of glucose + fructose to make sucrose. Glucose is reacting utilizing its hydroxyl group with fructose to yield one sucrose.

Fructose is what carbohydrate chemists call "non-reducing" while sugars like glucose are "reducing" sugars. We can also use that nomenclature with disaccharide like sucrose, maltose, etc. Sucrose has no reactive hydroxyl group open so it is called a non-reducing disaccharide. Maltose (two glucoses together) does have one hydroxyl and can polymerize (as in the case of starches).

daniel.kurz wrote:The short answer is that is unfavorable for this to happen. Glucose does polymerize because it is favorable.

The energetics of the reaction are favorable because glucose has a reactive hydroxyl group that is made reactive by the fact that glucose in the 5 carbon ring with an oxygen bound up is a hemiacetal. This make the hydroxyl group reactive and thus it reacts with another carbohydrate with the loss of the hydroxyl group from the first carbohydrate and a hydrogen from the second carbohydrate to yield water (classic condensation reaction). The fructose in question is an acetal and thus only has no reactive hydroxyl. Fructose can only be reacted with as in the classic case of glucose + fructose to make sucrose. Glucose is reacting utilizing its hydroxyl group with fructose to yield one sucrose.

Fructose is what carbohydrate chemists call "non-reducing" while sugars like glucose are "reducing" sugars. We can also use that nomenclature with disaccharide like sucrose, maltose, etc. Sucrose has no reactive hydroxyl group open so it is called a non-reducing disaccharide. Maltose (two glucoses together) does have one hydroxyl and can polymerize (as in the case of starches).

So I went back to my biochemistry textbook to find the exact wording that will satisfy everyone.

I was using an incomplete explanation. The way that it goes is that the anomeric carbon (the carbon adjacent to the oxygen in the sugar ring is the one that makes the ketone group in the open ring form and the oxygen of the ketone is the oxygen in the ring) is bound up into a non-reducing disaccharide as is the case sucrose. This anomeric carbon has the reactive hydroxyl group that I have referred to that is used in the reducing part and makes the bond to the monomer or ogliosacchride that is being bound to. This reaction goes by the condensation reaction of hydroxyl group lost from the fructose and oxygen from the sugar it is binding to. This ties up the anomeric carbon in the bond. There are no other anomeric carbons in fructose with reactive reducing hydroxyl groups. Thus the fructose cannot bind to another molecule once it has bound itself up. The other hydroxyl groups can be reacted with in the lab if forced but in nature steric interactions between sugars prevent those other hydroxyl groups from being used. In other sugars like maltose and cellulose steric interactions don't exist and so there are additional hydroxyl groups that can be used. Does that make sense now?

Please see the pictures that I have included. Both pictures come from Lehninger's Biochemistry 4th edition.

You will see in this picture below the reactive hydroxyl group is bound up in the disaccharide which removes it from reacting further. Other hydroxyl groups cannot be used because of steric interactions that are repulsive.

2.jpg (29.93 KiB) Viewed 12385 times

You will notice that the sugars bind end to end to eliminate steric hinderance and there is a free reducing sugar still there to further react.

By the way the reason why fructose can't bind to each other is the steric interaction between those large CH2OH groups attached to the anomeric carbon. You can force it in the lab, but its very unstable and the yield isn't good.